Tsitsipas, Zverev coast into Monte Carlo quarter-finals
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Tsitsipas dispatched Serbian Laslo Djere 7-5, 7-6 (7/1) with second seed Zverev easing past Spaniard Pablo Carreno Busta 6-2, 7-5.
Tsitsipas, the world number five, will play Diego Schwartzman for a place in the semi-final after the Argentine 12th seed rallied past Italy's Lorenzo Musetti 2-6, 6-4, 6-3.
"I had to put in a very good performance," said Tsitsipas, who lost in five sets to Novak Djokovic in the French Open final last year.
"Laslo did not hand me victory, but I tried to stay 100 percent focused.
"The tie break was pivotal and I am delighted to have produced that level of tennis," added the 23-year-old Greek.
Zverev, the world number three, next plays either fifth seed Andrey Rublev, last year's runner-up, or Italian ninth seed Jannik Sinner.
"For the second clay-court match of the season, I can't complain too much," second seed Zverev said.
"Yes, I have lost focus a little bit in the second set, but at the end of the day, he's somebody that can really play. I'm happy with a two-set win."
"It's great because here it's a little bit slower I think than other clay courts that we play on, it is very important to get that confidence going on your serve," Zverev said.
"All around, it's important to get these kinds of wins."
Indian Wells champion Taylor Fritz saw off his doubles partner Sebastian Korda, the conqueror of recent Miami champion Carlos Alcaraz in the previous round, 7-6 (7/4), 7-5.
"It's crazy really, I have reached the quarter-finals in four of the last five Masters tournaments," said the 13th-ranked Fritz.
"If I had been luckier in Miami I could have been five from five. I am in a good place at the moment. I seem to be improving all the time."
Fritz will next play Alejandro Davidovich Fokina, the Spaniard who knocked out world number one Djokovic on Tuesday.
Davidovich Fokina progressed to the last-eight by beating the experienced Belgian David Goffin 6-4, 6-1 on Thursday.
"I carried the emotion of beating Djokovic into this match," said Davidovich Fokina.
"I knew it would be a tough match as David won in Marrakech last weekend.
"However, he appeared more tired than I was and I am delighted to have reached the quarter-finals."
There was an upset though as Norway's fourth-seed Casper Ruud followed Djokovic and Alcaraz out of the tournament.
The Miami finalist was beaten by Grigor Dimitrov of Bulgaria, 6-3, 7-5.